Plasma accelerator system



SARCH RW Sept 29, 1964 P. GLoERsl-:N ETAL PLASMA ACCELERATOR SYSTEM Filed Aug. 18, 1959 United States Patent O 3,151,259 PLASMA ACCELERATOR SYSTEM Per Gloersen, Levittown, Bernard Gorowitz, Elkins Park, and Kenneth G. Moses, Philadelphia, Pa., assignors to General Electric Company, a corporation of New York Filed Aug. 18, 1959, Ser. No. 834,574 6 Claims. (Cl. 310-11) This invention relates to plasma production and acceleration and has for an object the provision of a pulsed plasma accelerator system for producing electromagnetic acceleration of a low-density gas. Plasma may be defined as gas in an ionized state in which substantially equal numbers of positive ions and electrons are present and hence is substantially neutral.

The present invention is concerned with plasma propulsion for vehicles in regions of extremely low pressure, such as outer space, such for example as plasma propulsion for spacecraft attitude control, orbit correction, orbit transfer or the like under powered flight or in final orbit. Because of the relatively small gravitational forces existing in outer space, comparatively small thrusts are adequate for moving relatively large masses. As hereinafter explained, the present invention provides substantially increased efficiency both in conversion of the electromagnetic energy to kinetic energy of the gas by utilizing a diverging magnetic ield of substantial length and in power conversion by making the accelerator self-triggering.

The present invention utilizes the combination of two factors which are operative in expelling the plasma. The lirst of these may be described as a magnetohydrodynamic pressure effect. The second factor is due to ohmic heating of the plasma.

In accordance with one aspect of the invention, there is provided a method of producing plasma propulsion wherein a radial electric field is produced within an evacuated gap between concentric electrodes. A magnetic eld is produced axially of the gap in a manner such that it diverges throughout the axial length of the electrodes. Pulses of low-density gas are periodically introduced into the gap and a current is caused to pass between the electrodes in timed relation with introduction of the gas pulses to create a unidirectional llow of' plasma axially of the gap.

Further in accordance with the invention, there is provided a plasma accelerator having a conical electrode and a cylindrical electrode surrounding the conical electrode. The electrodes are supported in spaced relation with the vertex of the conical electrode being disposed towards the nozzle end of the accelerator to provide a diverging gap. The accelerator includes means for introducing a pulse of gas into the throat end of the gap. A coil surrounds the cylindrical electrode and is connected in series with the gap so that the accelerator will be selftriggering upon admission of each pulse of gas into the otherwise evacuated gap which is evacuated under normal operating conditions in outer space.

In accordance with a further aspect of the invention there is provided in a plasma accelerator the sub-combination of an electrode having a passage therethrough and a valve plate disposed over the passage. A sealing means in the form of a ring surrounds the passage and is disposed between the electrode and the valve plate. The valve plate is normally held against the sealing means by means of a spring for producing a biasing force on the valve plate to close the passage in the electrode. A coil surrounds the passage and is disposed adjacent the valve plate for producing a repulsion force on the plate to move it away from the passage in opposition to the 3,151,259 Patented Sept. 29, 1964 biasing force so as to permit gas to enter the passage through the electrode.

For a more detailed disclosure of the invention and for further objects and advantages thereof, reference is to be had to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic view of a system embodying the present invention; and

FIG. 2 is an explanatory view showing the invention as applied to spacecraft attitude control or the like.

Referring to FIG. 1 the plasma accelerator system 10 includes an electrode 11 which is conical in shape and a cylindrical electrode 12 which surrounds the conical electrode 11 and forms a radial gap therebetween. The electrodes 11 and 12, which are made from suitable metal such as copper or brass, are adapted to be supported in spaced relation by any suitable means and the outer electrode 12 is surrounded by an electric coil or winding 13 having a plurality of turns. The coil 13 is electrically insulated from the cylindrical electrode 12 by suitable means such as a glass sleeve 14.

In operation, at least the electrode-gap of the accelerator 10 is adapted to be subjected to very low pressures. This condition, normally present in outer space, may be produced for the purposes of laboratory testing by suitable means, such as a vacuum pumping system providing an evacuated region indicated by phantom lines 15.

The accelerator 10 is provided with a D.C. power supply 16 and in the preferred embodiment, the driving coil 13 and the radial discharge gap between electrodes 11 and 12 are connected in series. One or more capacitors 18 are adapted to be charged by the power supply 16, and with it provide a suitable high-voltage, high-current source for the accelerator 10. The power supply 16 for charging the capacitor 18 may be of any suitable high-voltage D.C. type and may include a current limiting resistor or it may provide pulse-charging through an inductor, the latter arrangement being desirable, since it substantially eliminates ohmic heating losses in the charging circuit. The capacitors 18 are adapted to be discharged across the radial gap. The resulting current ilow, as indicated by I, is through conductor 21 along the inner electrode 11 across the radial gap to the cylindrical electrode 12 and thence by way of the metal-supporting flange 22 through the coil 13, and back through conductor 23. With circuit parameters affording critical damping, the discharge current traversing the gap and exciting the coil 13 is a single unidirectional pulse.

The shape of the electrodes 11 and 12 and the position of the coil 13 are such that two forces will be operative in expelling the plasma from the discharge end of the accelerator into outer space or other evacuated region 15. The rst of these is in the nature of a magnetohydrodynamic pressure and is created in an axial direction by reason of the interaction of both the magnetic eld BZ due to passage of current through the coil 13 and the field B6 due to three other currents with the plasma created between the electrodes. These are (l) the axial currents in both electrodes 11 and 12; (2) the ring current in the plasma caused by the v BZ electric field where v is the velocity of the radial motion of the charge carriers in the plasma; .and (3) the ring current induced in the plasma by the time variation of the magnetic eld BZ. Magnetohydrodynamic effects are described in a publication entitled Magnetohydrodynamics by Cowling and in a publication entitled Physics of Fully Ionized Gases by Spitzer, both of which are published by Interscience Publishers, Inc.

It is important that the magnetic field BZ diverge throughout the entire region occupied by the plasma in order that the magnetic energy may be coupled to all of the plasma for optimum efliciency. If the magnetic field were diverging in only a portion of such region7 then only a corresponding portion of the plasma would be accelerated by this means resulting in a very low efficiency typical of prior accelerators.

The second force which contributes to expelling the plasma from the accelerator is due to the pressure gradients on the gas derived from the ohmic heating due to the currents induced in the plasma by the time-varying magnetic field BZ and Ba and to the radial discharge current.

The electrode 11 is provided with an internal passage 11a which extends axially of the electrode l1 and terminates in a series of small radial ports 11b which extend around the circumference of the electrode 11 near the throat end of the axially diverging gap between the electrodes. A suitable gas such as nitrogen, argon, hydrogen or helium is adapted to be introduced into the radial gap from passage 11a by way of a valve 25. Air may also be used as a fuel; however, small mass materials, such as hydrogen or helium, are accelerated more easily than heavier ones. Since the principle of the accelerator is to provide a large exhaust momentum by providing a very large velocity to a small mass, the use of low atomic mass material is preferred. The valve 25 comprises a housing 26 which is adapted to be supported at the rear of the electrode 11. The axial passage 11a is adapted to be surrounded by a field coil 28. The housing 26 is adapted to be mounted on the end of the electrode l1 and preferably is divided into two sections 26a and 26!) which are sealed together by a rubber gasket 29. A spring 35 supports a valve plate or disc 30 in centered relation with respect to the passage 11a in electrode lll. The valve seat around the passage 11a is provided by a sealing member 32 which is disposed within a groove 33. The sealing member 32 is preferably in the form of an O-ring made from Teflon and the valve plate 30 is adapted to be held against the sealing ring 32, by means of a biasing force produced by the coil spring 35. The valve plate 30 is shown in the form of a round disc and is momentarily opened by pulseexciting the coil 28. This induces eddy currents in the plate 30 causing a repeated repulsion of the plate 30 away from the coil 28 and seat 32.

In a typical embodiment, the field coil 28 of the valve was made from copper ribbon and embedded in porcelain cement to prevent electrical breakdown and to avoid deformation and movement due to its internal magnetic fields. The valve plate 30 was made from highly polished brass to which was soldered the spring 3S which is made from steel. The valve seat 32 was a Tefion O-ring and the groove 32 was highly polished. The energy for the valve opening was provided by a power supply 459 which included a l kv., 3 ufd. G E. capacitor, charged from 4.5-8 kv. Its discharge was triggered by a three-electrode air gap whose breakdown was initiated by the timed spark of an kv. pulse generator.

From the above description, it will be recalled that the radial gap and the driving coil f3 were connected in series circuit with the power supply i6. With this arrangement, and by introducing the gas through ports 1lb in the radial gap between the electrodes lll and 12, the accelerator i@ is trigged automatically and internally and thus there is eliminated the external triggering losses due to the energy used in external switching. The valve permits the gas to be introduced in pulses which may be controlled to a very short duration of substantially less than microseconds. Since the evacuated gap of accelerator 10 is capable of holding off the potential of the charged capacitors 18, the pulsed admission of the gas provides a necessary ionizing medium to trigger the discharge and produce a mass of the plasma which is expelled axially to the right of the accelerator 10.

A plurality of accelerators are adapted to be mounted on a space vehicle 50 as shown in FIG. 2. The position of the vehicle 5G may be changed with respect to its normal flight path by operating one or more of its plasma 4 accelerators 10 to obtain the required corrective impulse. As shown, the vehicle 50 has changed its fiight path from path I to path II.

While there has been described a preferred embodiment of the invention, it will be understood that further modifications may be made without departing from the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

l. A method of producing plasma propulsion for spacecraft attitude control or the like, comprising producing a radial electric field with an evacuated gap between concentric electrodes, producing a magnetic field axially of said gap, said magnetic field diverging throughout the axial length of the electrodes, periodically introducing pulses of low-density gas into said gap, and causing a current to pass between said electrodes in timed relation with introduction of said gas pulses to create a fiow of plasma axially of said gap and in the direction of its divergence.

2. The method according to claim 1 characterized by introducing the gas pulses at a location between the electrodes and connecting the gap in series with the means for producing the magnetic field so that said system will be self-triggering upon admission of a pulse of the gas into the evacuated gap.

3. A system for producing plasma propulsion for spacecraft attitude control or the like comprising a pair of spaced concentric electrodes forming a radial gap which is adapted to be evacuated, means producing a radial electric field within said gap, means for producing a magnetic field axially of said gap, said concentric electrodes being so constructed and arranged that said magnetic field diverges throughout the axial length of said electrodes, means for periodically introducing pulses of low-density gas into said radial gap, and means for causing a current to pass between said electrodes in timed relation with introduction of said gas pulses to create a fiow of plasma axially of said gap.

4. A system according to claim 3 wherein said means for introducing gas pulses is disposed at a location between said electrodes, and said gap is connected in series circuit with said means for producing said magnetic field so that said system is self-triggering upon admission of a pulse of said gas into said evacuated gap.

5. A plasma accelerator comprising a conical electrode, a cylindrical electrode surrounding said conical electrode, said electrodes being supported in spaced relation with the vertex of said conical electrode being disposed towards the nozzle end of said accelerator to provide a diverging gap, a coil surrounding said cylindrical electrode, means for introducing a pulse of gas into said gap, a source of high voltage and high current, and means for connecting said gap and said coil in series circuit with said source so that said accelerator will be self-triggering upon admission of a pulse of gas into said gap.

6. A plasma accelerator comprising; a first electrode having a conical portion and a support portion; an internal passage formed in the support portion extending axially of the first electrode, said passage terminating in a plurality of small radial ports which are located near the base of the conical portion of the first electrode; a cylindrical electrode surrounding the conical portion of said first electrode; means supporting said electrodes in spaced relation to form a diverging gap between the inner surface of the cylindrical electrode and the exterior surface of the conical portion of the first electrode; valve means for periodically introducing a pulse of gas into the gap between said electrodes, said valve means comprising a valve plate made of a conductive material disposed over the passage in the first electrode, sealing means surrounding said passage and disposed between the first electrode and the valve plate, means for producing a biasing force on the valve plate to hold the plate against the sealing means to close said passage, a coil disposed adjacent said valve plate, said coil adapted to be pulsed to force the valve plate from the sealing means; a coil surrounding the cylindrical electrode; circuit means, adapted to be connected to a source of electrical power, for connecting the rst electrode, the cylindrical electrode, and the coil around the cylindrical electrode in series; whereby, upon the admission of a pulse of gas into said gap, current may flow through said circuit, the current flow between the rst electrode and the cylindrical electrode being through the gas introduced in the gap between the electrodes.

References Cited in the le of this patent UNITED STATES PATENTS Rupp June 27, 1933 Brown Sept. 25, 1934 Hardway July 7, 1953 Bremer et al Aug. 24, 1954 Ray Dec. 21, 1954 FOREIGN PATENTS Russia Jan. 22, 1958 Great Britain Feb. 9, 1955 Great Britain May 7, 1958 

1. A METHOD OF PRODUCING PLASMA PROPULSION FOR SPACECRAFT ATTITUDE CONTROL OR THE LIKE, COMPRISING PRODUCING A RADIAL ELECTRIC FIELD WITH AN EVACUATED GAP BETWEEN CONCENTRIC ELECTRODES, PRODUCING A MAGNETIC FIELD AXIALLY OF SAID GAP, SAID MAGNETIC FIELD DIVERGING THROUGHOUT THE AXIAL LENGTH OF THE ELECTRODES, PERIODICALLY INTRODUCING PULSES OF LOW-DENSITY GAS INTO SAID GAP, AND CAUSING A CURRENT TO PASS BETWEEN SAID ELECTRODES IN TIMED RELATION WITH INTRODUCTION OF SAID GAS PULSES TO CREATE A FLOW OF 